1. Field of the Invention
The invention relates to an infrared telescope comprising means to harmonize its input and output axes. It may be recalled that the telescope is a device used to observe a distant scene. The telescope gives an image of the scene at infinity. By analogy, an infrared telescope enables the observation of an infrared scene and gives an image thereof that is visible at infinity.
Many sighting systems such as for example a gun sighting device, a laser telemeter or a missile firing station use a telescope working in the visible light spectrum. This telescope, called a day telescope, defines an axis by means of a reticule. It may be necessary to make these sighting systems work in a frequency spectrum other than the visible spectrum, such as for example the infrared spectrum. A usual frequency band of the infrared spectrum ranges for example between about three and five .mu.m. Another band commonly used in the infrared spectrum is located approximately between eight and twelve .mu.m. The invention of course is not limited to these frequency bands. To make these sighting systems work at such frequencies, advantageous use is made of an infrared telescope with a magnification of unity. The infrared telescope is placed before this day telescope.
2. Description of the Prior Art
Embodiments of this type of telescope exist but they nevertheless have a particular difficulty. It is indeed important to harmonize the input and output optical axes of the infrared telescope in such a way that an observer can look at the same scene with or without the infrared telescope. In particular, the aiming axis defined by the reticule of the day telescope must be preserved when an infrared telescope is placed before this day telescope. In the particular case of an infrared telescope placed before the day telescope, the input and output axes of the infrared telescope have to be parallel. Harmonizing the input and output axes of a telescope of this kind implies reducing possible defects of parallelism of these axes, ideally so as to cancel these defects or, at least, to bring them within acceptable tolerances. The invention of course is not limited to a telescope comprising parallel axes. It can also be implemented for a telescope whose input and output axes are not parallel. Harmonizing axes, in this case, means limiting their defects of relative position.
The example of an infrared telescope with a magnification of unity has being chosen for a better understanding of the problem raised, but it is clear that the invention is not limited to this type of telescope. The invention can be implemented for any type of telescope whatever its magnification.
It is an aim of the invention to harmonize the input and output axes of the telescope with a precision that is compatible with the application envisaged. This precision may, for example, be lower than 100 .mu.rd.